The present invention relates generally to aqueous coating compositions comprising generally lipophilic, crosslinkable binders which are dispersed in an aqueous medium with the aid of a special oligomeric/polymeric amphiphatic reactive emulsifying agent.
Coating compositions comprising generally lipophilic, crosslinkable binders are well-known in the art. Because the binders are lipophilic, these coating compositions are most readily formulated as organic solvent based systems. Increasingly stringent health/safety and environmental legislation, however, is making the use of organic solvents an expensive and unfashionable option for the coatings industry. As such, the industry is intensively seeking waterborne alternatives to their long-standing and commercially successful organic solvent based products; however, it has been difficult at best to make the switch to water without significant loss of coating stability and/or resulting film quality.
One pursued option has been to render the lipophilic binders water-dilutable (so-called "self-emulsifying" resins). This can and has been done, for example, by building hydrophilic functionality into a polymeric backbone or by providing a polymer structure wherein this hydrophilic functionality is made pendant to such polymeric backbone. See, for example, U.S. Pat. No. 2,634,245, U.S. Pat. No. 3,379,548, U.S. Pat. No. 4,028,313, U.S. Pat. No. 4,315,044, U.S. Pat. No. 4,321,305, U.S. Pat. No. 4,608,406, U.S. Pat. No. 5,021,544 and EP-A-0355892. These self-emulsifying resins, however, generally possess a relatively large total number of hydrophilic groups which tends to render the resulting films somewhat water sensitive.
In order to overcome these disadvantages, attempts have been made to disperse these lipophilic binders in water with the aid of apart dispersing agents. For example, U.S. Pat. No. 4,352,898, U.S. Pat. No. 4,598,108, U.S. Pat. No. 4,737,530, U.S. Pat. No. 4,929,661 and EP-A-0262720 disclose the use of relatively low molecular weight organic compounds as dispersing agents. The described aqueous coating compositions, while satisfactory in many respects, may still require the use of significant amounts of organic cosolvent. Additionally, the described dispersants may evaporate and contribute to the overall VOC, and what dispersant remains in the resulting film tends to act as an undesirable hydrophilic center.
Relatively higher molecular weight surfactants have also been used. For example, U.S. Pat. No. 3,945,964, U.S. Pat. No. 4,318,832 and WO84/00169 describe the use of various non-ionic (e.g., polyoxyalkylene) and anionic surfactants. The described surfactants, however, are generally non-reactive and, while solving certain disadvantages of the lower molecular weight organic compounds, have a stronger tendency to remain as undesirable hydrophilic centers in the resulting films.
In still a further attempt to overcome the problems associated with the above-referenced systems, it has been proposed to use surfactants which become bound into the film structure during the crosslinking reaction. Such "reactive" emulsifiers have been described, for example, in AU-A-82247/87, GB-A-2100271, DE-OS-2455896, DE-OS-3900257, U.S. Pat. No. 3,979,346, U.S. Pat. No. 4,221,685 and U.S. Pat. No. 4,233,194.